1. Field of the Invention
The invention relates to a voltage regulator circuit using a Zener diode to produce a stable reference voltage, comparison means to compare a supply voltage with this reference voltage and regulation means to limit the supply voltage accordingly. It can be applied for example to the limiting of the value of the output voltage of a voltage-multiplier circuit, for example within a programmable electrical memory.
2. Discussion of the Related Art
In many electronic applications, it becomes necessary to use circuits to produce analog supply voltages suited to the proper working of the application. It may thus be necessary, starting from a basic supply which may be a battery or a mains supply, to develop circuits used to reduce or increase the basic supply voltage available. For example, if a microprocessor of a computer connected to the mains is supplied directly with 220 volts it will be destroyed. On the contrary, if it is desired to make a high-impedance measuring instrument supplied by a battery, it will be necessary to develop a switching regulator in order to have sufficient measuring currents available. A given circuit could also be formed by different modules requiring the production of supply voltages proper to these modules. For example, if it is desired to supply a microcontroller formed by at least one microprocessor and one read-only memory, it might be necessary to produce supply voltages of different values for the microprocessor and the memory.
Indeed, if we consider a MOS type memory organized in matrix form with cells formed by MOS transistors, it could be necessary to provide supply circuits internal to this memory to produce specific voltages in order to bias the control gates of these transistors. Typically, supply voltages greater than the rated voltage of the circuit (of the order of 3 to 5 volts) will be produced by means of voltage multipliers.
Conventionally, a voltage multiplier circuit (also called a load pump) is used to produce an output voltage from a received supply voltage by charging and discharging one or more capacitors consecutively, in such a way that the output voltage is greater than the received supply voltage. The output voltage increases gradually to reach an asymptotic value that is typically equal to a multiple of the received supply voltage. For example, to produce an output voltage equal to twice the received voltage, it is possible to use a circuit called a "Schenkel doubler". One drawback of this type of circuit is the fact that the output voltage increases ever less rapidly with time. In order to reduce the time needed to reach the desired output voltage, therefore, it becomes necessary to oversize the multiplier and to limit it at output. For example, instead of using a circuit that doubles the received voltage, the circuit used is one that triples it and the output voltage is limited to twice the voltage received once this value is reached.
Thus, to limit the value of the output voltage, it is possible typically to use a parallel regulator. The regulators contain three basic elements: a precision reference voltage, an error amplifier and a power control element. The different sub-units of a regulator are connected in a negative feedback loop in such a way that the output voltage is controlled and remains constant.
The simplest regulator consists of a Zener diode parallel-connected to a load supplied by the output voltage. The term "load" is understood to mean, for example, the capacitance equivalent to a set of control gates of MOS transistors of a memory. One drawback of this type of regulator is that the output voltage is set so as to be equal to the reverse voltage of a Zener diode or to a multiple of this reverse voltage (in the case of several series-connected diodes). Thus, the voltage cannot be limited to a value lower than that of the reverse voltage of the Zener diode. Indeed, the diode has to be supplied with a voltage greater than this reverse voltage to obtain this reference value. Now, it is increasingly sought to develop products that work with a low supply voltage, of the order of 3 volts. If it is desired, for example, to produce a voltage of 4.5 volts, it would not be possible to use a diode whose reverse voltage value is 5.5 volts in the current state of the art.
Conventionally, when the desired output voltage is greater than the reverse voltage of a reference Zener diode, there also exist known regulators where a transistor controlled by the output of a comparator is parallel-connected to the load. At an input of this comparator, the reverse voltage of a reference Zener diode is compared with a fraction of the output voltage picked up by means of a voltage divider bridge.
An arrangement might be designed where this system is adapted by dividing the reference voltage in this way. This approach is not acceptable. For, it requires a resistive chain in parallel with a load, and when the load is capacitive, this increases the build-up time of the output voltage.
Further, it is not possible to make direct comparisons of the reverse voltage and the voltage to be limited if the voltage is to be limited to a value below the value of the reverse voltage and, secondly, even if the opposite is the case, since a comparator cannot be used for this purpose if the supply voltage available for the comparator is lower than the voltages to be compared.
In view of the foregoing, the aim of the present invention is to propose a voltage regulation circuit that can be used to limit a voltage to a value below a reference voltage, these two voltages being greater than a supply voltage.